As you know, the gas tank on these cars sits on a ledge behind the back seat and, if the carb float valve sticks open, gas will flow into the carb, engine, out on the pavement, etc., unchecked. From what you describe, it sounds like this was not your problem but, as have others, I put a check valve (electric solenoid) under the car at the tank outlet and wired it (through a concealed barrel switch) into the ignition switch. If the barrel switch is off and the car is stolen and hotwire started, it will not be driven far. See Avanti Magazine, issue #170, pg. 48 for specifics. Good luck!